Solar panels on water canals seem like a no-brainer. So why aren't they widespread?

It looks to me like a completely unnecessary solution. Covering up the water is a good idea, but find an immediate and cost affective way to do it. Loads of custom ineffectively laid out solar panels is not that.

It's not as though we have a surplus of solar panels that need a place to go. If the money is being invested, place them in a location with them oriented toward the sun and not above water.

Yeah just makes sense, they just need some way to funnel rain water into them too still.

That itself is not hard either.

Dams too, rather then those little black balls ontop of the water a large solar array could help stop evaporation while still letting water collection happen